Annulenes, Heterocyclic and Polycyclic Aromatic Compounds
- Annulenes are monocyclic compounds containing alternating ring double bonds, such as benzene, but of different sizes. According to systematic nomenclature, benzene is a annulene, while cyclobutadiene is annulene, whereas cyclooctatetraene is an annulene, for instance.
Though annulene possesses 4n+2 π electrons, it is not aromatic because the ring is not plane, due to bonding angle strains. Nevertheless, annulenes of large ring sizes may also be aromatic. Annulene, for instance, is aromatic. Its carbon-carbon bond lengths are of nearly the same size and it posseses 4n+2 π electrons (with n = 4).